Global Environmental Change Research Pathways for the Next Decade (1999) / Chapter Skim
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6 Paleoclimate Overview
6 Paleoclimate Overview
Pages 237-292
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From page 237... ...
Earth history provides a unique opportunity to assess the temporal and spatial characteristics of climate variability prior to any anthropogenic forcing; assess the natural rates of change associated with the evolution of the Earth system to understand how physical and biospheric systems interact across multiple time- and space scales; define the nature of the sensitivity of the Earth's climate and biosphere to a large number of forcing factors; examine the integrated climatic, chemical, and biological response of the Earth system to a variety of perturbations; and test the predictions of numerical models for conditions significantly different from the present day. In effect, the paleoclimate record provides a series of cases and lessons upon which our understanding of climate change can be constructed and tested.
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From page 238... ...
New advances in paleoclimate research reaffirm the necessity to view climate change over varying timescales; utilize a variety of globally distributed paleoclimate records that monitor change throughout the Earth system; and focus attention on well-dated, highly resolved multivariate paleoclimate records. These paleodata are essential for understanding global environmental change and its potential impact on humans, assessing human influence on the global environment and for the evaluation of predictive climate models.
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From page 239... ...
While remarkable efforts are under way to resolve the history and significance of the human influences on climate, pollution, and resource depletion, our understanding of climate change is still hampered by a lack of knowledge of the processes that underlie natural climate variations. The importance of understanding natural climate variability has been clearly articulated in previous National Research Council (NRC)
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From page 240... ...
While the causes of rapid climate change events and natural climate variability, in general, are still not fully understood, evidence continues to accumulate emphasizing the significance of a variety of climate processes, such as changes in thermohaline circulation of the world's oceans, Earth's orbitally induced (Milankovitch) cycles of insolation, solar variability, greenhouse gases, volcanic activity, and ice sheet dynamics.
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From page 241... ...
Orbitally induced variations in insolation at the Milankovitch periods (primarily 100,000, 41,000 and 23,000 years) explain much of the change in global ice volume throughout the late Pleistocene and have been identified in a variety of paleoclimate records (e.g., marine and ice cores and loess sequences)
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From page 242... ...
The central Greenland ice cores provide a framework for other paleoclimate records because of their relatively precise dating. The current best estimate of the age at ~2,800 m is ~110,000 years, based on a combination of multiparameter annual layer counting combined with measurements of the d18O of atmospheric O2 calibrated with the Vostok ice core in Antarctica.5 Error estimates in the dating are quite remarkable, from 2 percent for 0 to11,640 years ago to 10 percent for over 40,000 years.6 Agreement between the GISP2 and GRIP ice cores (separated by 30 km or ~10 ice thicknesses)
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From page 243... ...
.12 Paleoclimate records from North Atlantic marine sediment cores also contain notable millennial-scale variability,13 although the exact timing of these events is less precisely known than for the Greenland ice cores. Several of the marine cores reveal evidence that the formation of NADW (North Atlantic deep water; warm, saline, nutrient-depleted deep return flow water)
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From page 244... ...
anic thermohaline circulation, fluctuated dramatically in the past.14 NADW diminished greatly during the last glaciation and was relatively strong during the interglacials. Recent studies confirm that NADW fluctuates on millennial timescales and correlates with sea surface and atmospheric temperatures.15 Changes in the flux of ice-rafted detritus, d18O of foraminifera shells, and the abundance of climate-sensitive foraminifera indicate that during the last glaciation the North Atlantic was punctuated by iceberg discharge events potentially produced in response to changes in ice sheet dynamics.16 The largest of these (Heinrich events)
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From page 245... ...
PALEOCLIMATE OVERVIEW 245 FIGURE 6.4 Top: GRIP ice core values for methane. The thick line runs through the mean concentration (black dots)
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From page 246... ...
climate records covering the last glacial/interglacial cycle. Top: Plot shows the close correlation between GISP2 and Vostok δ18O of O 2 in air in these ice cores.
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From page 247... ...
The GISP2 timescale was transferred into the GRIP record at the sharp interstadial boundaries, which are precisely located in both ice core records (Dansgaard et al., 1993; Mayewski et al., 1994) , and then ages were interpolated between these boundaries.
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From page 248... ...
Abrupt changes in atmospheric circulation patterns and precipitation regime also are recorded over eastern Asia in a thick sequence of wind-deposited loess from central China.19 Records of alpine glacier fluctuations, mountain snowlines, and paleovegetation in the Andes reveal climate fluctuations that are similar in regularity to events in the Greenland ice cores.20 While the exact phasing of rapid climate change events from region to region is still being examined, new advances in age-dating correlation techniques have provided insight into the bipolar phasing of major climate events close to the last glacial maximum. Measurements of the d18O of atmospheric O2 from the Byrd and Vostok ice cores in Antarctica and the GISP2 ice core suggest that the transition from glacial maximum to deglaciation began in Antarctica approximately 3,000 years before the onset of warming in Greenland.21 This view creates a more complex event phasing than that suggested by previous correlations of marine, coral reef, and ice extent records, which suggested that during the last termination nearly synchronous temperature changes affected ice masses from the poles to the equator.22 New advances in paleoclimate reconstruction also come from the tropics.
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From page 249... ...
. The PCI provides a relative measure of the average size and intensity of polar atmospheric circulation.
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From page 250... ...
Millennial-scale and finer Holocene climate fluctuations have been identified for more than two decades in a variety of Holocene records.29 In general, however, Holocene climate variability is significantly more subdued in magnitude than that recorded during the last glaciation, and significantly less attention has been paid to this portion of the paleoclimate record. New Developments Environmental response to climate change since the last glacial maximum has been considerable.
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From page 251... ...
Some attempts have been made to simulate vegetation patterns that could exist in 2 x CO2 eastern North America, utilizing general circulation models (GCMs) coupled with paleoclimate-vegetation distributions.33 Annually resolved continuous paleoclimate records from the GISP2 ice core demonstrate that Holocene climate is characterized by annual- to millennial-scale variability and that Holocene climate is significantly more complex than glacialage climate.34 Time series for the major ions dissolved in the atmosphere, utilized as tracers for major atmospheric circulation systems, reveal a strong association between expansions of northern hemisphere polar atmospheric circulation systems and a variety of discontinuous paleoclimate records that record worldwide coolings (see Figure 6.4)
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From page 252... ...
800 750 .22 700 .2 650 600 550 -34.8 GRIP δ18O (0/00) -35.2 world wide glacial -35.6 expansions and their relative magnitiude -36 synthesis of Europe and Greeland climate proxy records, showing North America cold periods Cockburn Moraine Southern Hemisphere Advance YD event-CISP2 0 2000 4000 6000 8000 10,000 12,000 Calendar Years beforePresent 0 2000 4000 6000 8000 10,000 14CYears Before Present FIGURE 6.8 From top to bottom: GISP2 annually dated Holocene EOF1, a proxy for northern hemisphere polar cell intensity (PCI; described in Figure 6.8)
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From page 253... ...
Mid-Holocene decreases in monsoon strength appear to have been more time transgressive than the earlier abrupt increases and also more in phase with insolation forcing. The figure shows the timing of the first abrupt increase in monsoon strength (mean, 11,400 years)
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From page 254... ...
changes in the concentrations of volcanic aerosols and dusts.46 A variety of paleorecords are available to test the impact of these forcing mechanisms, including, for example, potential proxies for solar variability derived from d14C series in tree rings and 10Be series from ice cores, CO2 from ice cores, CH4 from ice cores, and volcanic sulfate from ice cores.47 The Late Holocene (~2,000 years ago to present) Summary of Previous Work Although the exact timing and geographic distribution of Holocene climate change events are complex, the past 1,000 to 2,000 years offer important opportunities for unraveling the decadal- to centennial-scale and finer climate variability that influences modern climate.
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From page 255... ...
suggest that fewer such events occurred during the MWP than during colder intervals prior to and following this time.56 Studies conducted over only the past 500 years, which do not include the LIA/MWP transition, suggest that El Niño recurrence rate is stationary over the long term but that strong El Niño events are nonstationary over centennial scales.57 Analysis of records covering the past 500 years suggests the presence of persistent natural interdecadal and century-scale climate oscillations. A compilation of paleoclimate records representative of the past 400 years of circum-Arctic climate variability indicates that the highest temperatures over this period have occurred since 1840, demonstrating the role of natural climate variability and, as of 1920, the added climate influence of atmospheric trace gases.58 Multidecadal modes and step-function changes in precipitation, temperature, and wind regimes have been identified in a number of regions, ranging from the Intertropical Convergence Zone (ITCZ)
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From page 256... ...
These contributions introduce the potential of climate-vegetation feedbacks to substantially modify the sensitivity of the Earth's climate to a large number of different forcing factors. Earth history enables an assessment of the importance of vegetation-climate feedbacks for a diverse number of forcing factors (carbon dioxide, solar variations, orbital variations, sea level, changes in continental geometry)
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From page 257... ...
There are proxy ENSO time series available from a variety of records, including, for example, historical records, tropical corals, tropical ice cores, and polar ice cores.67 Of particular note are the newly emerging annually resolved coral records that through calibration with the instrumental record provide proxies for zonal winds and precipitation (see Figure 6.12) .68 North Atlantic Oscillation (NAO)
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From page 259... ...
, the Southern Oscillation Index (SOI; Wright, 1989) ; composite coral δ18O (plotted reverse; Cole and Fairbanks, 1990)
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From page 260... ...
. Bottom: CO2 mixing ratios since early last century from DEO8 and DEO8-2 Law Dome ice cores and the South Pole record (Keeling, 1991)
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From page 261... ...
Examples of volcanic events recorded in nss sulfate spikes are (1)
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From page 262... ...
Deriving sufficient climate model sensitivity from changes in the Earth's orbit to produce the observed glacial-interglacial cycles has been a long-standing problem. Evidently, climate-vegetation feedbacks are a
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From page 263... ...
Extensive published summaries of the vegetation distribution and history for the Last Glacial Maximum have been provided.81 Global Tertiary (past 65 million years) vegetation distribution maps and recent updates are available.82 Extensive references are given for the Eocene climate,83 and both phytogeographic maps and extensive references for the midCretaceous are available.84 However, actual applications of climate-vegetation models for past climates have been limited.
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From page 264... ...
on the leaves of land plants.88 It has also been suggested that C4 plants are adapted to conditions of water and CO2 stress and that their widespread expansion 5 million to 7 million years ago was related to lower carbon dioxide levels.89 These vegetation responses are related specifically to the nature of the climate forcing and introduce the potential of utilizing the floral record with proxies of past atmospheric carbon dioxide levels to examine plant responses. Warm Climates Summary of Previous Work Earth history is characterized by time periods in which the climate was significantly cooler and significantly warmer than the present day.
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From page 265... ...
According to one report,98 Eocene carbon dioxide levels may have been moderately higher than at present. The Cretaceous and Cenozoic warm climates present a substantial challenge to climate models used to predict future climate change.
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From page 266... ...
If the atmospheric circulation is weaker, the wind-driven ocean surface circulation should be weaker. The only apparent opportunity to increase poleward heat transport is through the deep thermohaline circulation of the ocean.
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From page 267... ...
The geological record also can be utilized to calibrate model sensitivity to increases in carbon dioxide concentrations in the atmosphere.103 For example, one study uses global, mean annual, equator-to-pole temperature gradients, ranges in atmospheric carbon dioxide, and assumptions about the nature of other forcing factors to suggest that the midrange of the IPCC estimates (1.5 to 4.5 degees) for a doubling of carbon dioxide is appropriate for much of the Earth's history.104 It was further noted that the uncertainties in some of these factors probably mean that the paleoestimates of climate sensitivity to increases in atmospheric carbon dioxide have about the same error range as the IPCC estimates.105 In more detailed studies using GCM results106 the actual distribution of surface temperatures was the basis for comparing model simulation results for differing levels of carbon dioxide.
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From page 268... ...
These consensus views have been expressed in several documents108 which form the basis for the specific scientific questions that follow.b Focus on the Past 2,000 Years Paleoclimate records demonstrate that Holocene climate has been by comparison with glacial climates both warm and relatively stable. Furthermore, it is within the confines of Holocene climate that modern civilization has emerged and prospered, which is suggestive of some relatively benign climate influence.
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From page 269... ...
. While these climate events do not serve as strict analogs for future warmer or colder climates because they predate the industrial era, they do offer important "climate opportunities." Furthermore, despite the fact that instrumental records are most commonly less than a century in length, abundant and relatively untapped records in the form of historical journals and natural archives (e.g., tree rings, ice cores, corals)
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From page 270... ...
Furthermore, despite their importance in climate forcing, changes in the concentration of greenhouse gases cannot fully explain documented changes in temperature. Rapid climate change events recently developed from the central Greenland ice cores and since found in marine and terrestrial sediments punctuate the slower pattern of ice sheet growth and decay.
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From page 271... ...
Phasing information will be crucial in determining the cause of these events. Changes in thermohaline circulation of the ocean are believed to play a major role in the production of rapid climate change events, but the causes of such changes are not well understood.
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From page 272... ...
emissions, changes in the hydrological cycle, volcanism, and ice sheet dynamics superimposed on insolation cycles must be investigated through both more wellresolved and well-dated paleoclimate records and modeling efforts to understand rapid climate change events. Glacial/interglacial cycles and rapid climate change events provide dramatic examples of the dynamic range of natural climate variability.
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From page 273... ...
Recognition of potential "surprises" in natural climate variability, such as rapid climate change events, ENSO, and drought recurrence, require long backward glances to assure proper perspective. As a corollary to this it is clear that the paleoclimate record is essential as a basis for quantifying signal to noise ratios in shorter observational records.
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From page 274... ...
A new standard for this type of record has been set by the Greenland ice cores, but too few such datasets exist. These records provide a new framework for examining shorter, less well resolved, discontinuous paleodata.
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From page 275... ...
Furthermore, through the hindsight offered by paleoclimate records, observing programs will be able to more accurately assess natural climate variability, patterns, trends, and signal to noise. Finally, paleoclimate records offer the climate modeling community a bold opportunity for testing climate response and forcing on a variety of timescales and resolutions heretofore untouched.
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From page 276... ...
While annually resolved records are available from tree rings and some ice cores, most paleoseries are dated by models, spot relative and absolute dating, and spot marker horizons. Great advances have been made in 14C dating of small-volume materials and the identification of unique events such as globally distributed volcanic events, but more work is needed.
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From page 277... ...
• Greenhouse gas concentrations and biogenic emissions are closely linked to temperature. • Rapid climate change events that operate at magnitudes and rates signifi cant to humans have operated throughout at least the past glacial/intergla cial cycle.
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From page 278... ...
(1994, 1997) ; paleoclimate records, Denton and Karlen, (1973)
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From page 279... ...
(1992) ; tropical ice cores, Thompson et al.
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From page 280... ...
1992. Detrital carbonate-rich sediments, northwestern Labrador Sea: Implications for ice sheet dynamics and ice-berg rafting (Heinrich)
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From page 281... ...
1993. The North Atlantic Oscillation signature in deuterium and deuterium excess signals in the Greenland Ice Sheet Project 2 ice core, 1840-1970.
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From page 282... ...
1993. Correlations between climate records from North Atlantic sediments and Greenland ice.
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From page 283... ...
1995. Sedimentary record of climatic variabil ity in the North Atlantic Ocean during the last glacial cycle.
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From page 284... ...
1993. Comparison of oxygen isotope records from the GISP2 and GRIP Greenland ice cores.
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From page 285... ...
1995. Global climate models and "dynamic" vegetation changes.
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From page 286... ...
1995. Rapid climate variability in the North Pacific Ocean during the past 95,000 years.
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From page 287... ...
1994. Changes in atmospheric circulation and ocean ice cover over the North Atlantic during the last 41,000 years.
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From page 288... ...
1996. Complexity of Holocene climate as reconstructed from a Greenland ice core.
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From page 289... ...
1981. The North Atlantic Ocean during the last glaciation.
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From page 290... ...
1993. Electrical conductivity measurements from the GISP2 and GRIP Greenland ice cores.
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From page 291... ...
1994. Record of volcanism since 7000 BC from the GISP2 Greenland ice core and implications for the volcano-climate system.
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